Transformer core reinforcing plate



Oct. 24, 1967 w. J. M NUTT ETAL TRANSFORMER CORE REINFORCING PLATE 2 Sheets-Sheet 1 Filed Aug. 31, 1965 (J 131311311111]. 0 0 O o O o o 0 o L L L L L L L L L LF. F:LT J

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WILLIAM J McNuTT WALLACE Filed Aug. 31', 1965 Oct. 24, 1967 w. J. MCNUTT ETAL 3,349,357

TRANSFORMER CORE REINFORCING PLATE 2 Sheets-Sheet 2 \llnlllllllll)! "I: I I mmur IN VENTORS.

WILL/AM J Mc/VuTT WALLACE M. JoH/vso/v ATTORNEY United States Patent 3 349 357 TRANSFORMER coizn hErNFoncrNo PLATE William J. McNutt and Wallace M. Johnson, Pittsfield, Mass., assignors to General Electric Company, a corporation of New York Filed Aug. 31, 1965, Ser. No. 483,912 4 Claims. (Cl. 336-210) ABSTRACT OF THE DISCLOSURE The spaced-apart upper and lower yoke clamps of a power transformer are connected by pairs of elongated reinforcing plates, each pair of plates lying flatwise along opposite sides of a laminated core leg and each plate being slotted longitudinally for a major portion of its length. Each slotted plate is provided with a plurality of crossarms which cooperate with clamping bolts through the adjacent core leg.

This invention relates to stationary electric induction apparatus, and more particularly to an improved core reinforcing plate for large electric power transformers of the laminated core type.

As described in Patent 2,910,6 63-Wilk and ONeil, large electric power transformers are commonly provided with a rectangular laminated steel core having several vertical legs in parallel spaced relation, joined together at opposite ends by a pair of horizontal yoke members. These laminated cores are clamped together by several means. One such clamping device consists of a pair of channel-shaped or box-shaped structural steel members lying along opposite sides of each yoke and clamped together by bolts through or closely adjacent the yoke laminations. The leg laminations must be both clamped together and relieved of tensile stresses resulting from any force tending to spread the yokes apart. For example, a structural tie between the yokes is necessary to resist short circuit forces and to permit lifting from the top yoke. In the larger sizes of transformers the leg laminations are often divided longitudinally and the legs assembled in two halves spaced apart by a central transverse slot. Thus it may be additionally necessary to tie or clamp together two separated halves of each leg. These purposes have been accomplished heretofore for each leg of the COre by means of a single pair of reinforcing plates formed of structural steel and laid along opposite sides of each leg parallel to the laminations. The reinforcing plates are clamped together by bolts through the leg, and are connected at opposite ends to the yoke clamps by means of end bolts or pins. Such a tie and clamping plate, i.e. reinforcing plate, is described in the foregoing patent.

We have discovered that in large power transformers, the leakageflux from each leg in planes generally transverse to the laminations is suflicient to heat conventional leg reinforcing plates to undesirably high temperatures. This is a result of eddy currents in the reinforcing plates generated by the radial component of leakage flux leaving the leg intermediate its ends and in directions passing through the reinforcing plate.

It is therefore an object of our invention to provide an improved core reinforcing plate for electric transformers. It is another object of the invention to provide an improved tie and clamping plate for the core legs of electric power transformers.

It is still further an object of our invention the provision of a combined leg clamp and yoke tie for power transformers which is not subject to excessive heating in operation.

It is a more 'particular object of our invention to provide a leg clamp and tie plate for power transformer cores in 3,349,357 Patented Oct. 24, 1967 which means are provided for minimizing undesired eddy currents in the plate while still meeting all other tying, clamping and lamination stacking functions required of such a plate.

In carrying out our invention in one preferred embodiment, we cannot between the spaced parallel yoke members of a transformer core an elongated flat bar of structural steel having a wide central longitudinal slot which terminates short of each end of the strip. To complete the reinforcing plate a plurality of rigid steel cross arms are fixed to the slotted bar in the slotted region, one cross arm being provided at the location of each core clamping bolt, and each cross arm extending across the slot and preferably beyond the sides of the slotted bar. To minimize eddy currents each cross arm is welded or otherwise conductively fixed to the slotted bar at only one side of the slot and insulated therefrom at the other side. Preferably one group of alternate cross arms is welded to the slotted bar at one side of the slot, and the remaining bars are welded at the other side of the slot.

Our invention will be more fully understood and its various objects and advantages further appreciated by referring now to the following detailed specification taken in conjunction with the accompanying drawing.

In the drawing:

FIG. 1 is a side elevational view of a three-legged transformer core embodying our invention and showing in phantom lines the location of windings upon the core legs;

FIG. 2 is an enlarged transverse cross-sectional view of the core taken along the line 2-2 of FIG. 1;

FIG. 3 is an enlarged plan view of a single core leg reinforcing plate of the type shown at FIGS. 1 and 2;

FIG. 4 is a transverse cross-sectional view of the core plate of FIG. 3 taken along the line 44 of that figure; and

FIG. 5 is a transverse cross-sectional view of one transformer core leg taken along the line 5-5 of FIG. 2.

FIGS. 6 and 7 are cross-sectional and plan views, respectively, of a core reinforcing plate of alternative structure to illustrate another embodiment of our invention.

Referring now to the drawing, and particularly to FIGS. 1 and 2, we have shown a three-legged rectangular power transformer core comprising three leg sections 1, 2 and 3 in parallel spaced relation, joined across their juxtaposed top and bottom ends by a pair of yoke sections 4 and 5, respectively, in parallel spaced relation. Each of the leg and yoke sections is formed as a laminated assembly of stacked sheets of magnetizable material, preferably grainoriented steel. The width of the steel laminations in each leg and yoke section is reduced in several steps from the center toward the outside of each core section to provide a cruciform cross-sectional configuration, such as shown at FIG. 5. All the leg and yoke sections are provided with at least one slot 6 (FIGS. 2 and 5) in the plane of the legs 1, 2 and 3 to provide for the circulation of cooling liquid through the core. The slot 6 is formed by the interposition of a plurality of spacers 7, and completely separates the front half of the core from the back half. If more than a single slot is provided in planes parallel to that of slot 6, the core will of course be divided into three or more parallel portions from front to back.

Each leg and yoke section is provided also with a central slot 8 (FIGS. 1 and 5) extending perpendicular to the plane of the three legs and formed by dividing and spacing apart each lamination in each leg or yoke section as the case may be. These transverse slots 8 serve to permit the circulation of cooling fluid, and in addition provide apertures for the passage of clamping bolts needed to hold the laminations together. Preferably the core is constructed with mitred joints at the corners where the leg and yoke laminations intersect, as illustrated at FIG. 1. A mitred joint of the interleaved overlapping type is suitable, as illustrated in Patent 2,792,554-Graham, but it will be understood that Without departing from the scope of our present invention, other types of core joints may be utilized. Similarly, any other core cross-sectional configurations may be used, with or without cooling slots through the core sections.

The laminations of the top and bottom yoke members 4 and 5 are clamped together between pairs of box shaped clamping members 10, a, and 11, 1111 respectively, made of strong structural steel. Each pair of yoke clamping members is connected together in spaced relation at its ends by intermediate transverse channel members 152. The transverse channels 12 are bolted in place tightly against the edges of the core laminations and serve to support the laminations during shipment and operation. The respective pairs of yoke clamps 10, 19a and 11, 11a are held in clamping relation to the yoke laminations by the transverse channels 12 and by a plurality of transverse bars 13 across the yoke clamps at the top and bottom outside the yoke lamination-s. The upper yoke clamps 11, 11a are provided with a pair of lifting lugs 14.

For clarity in illustration of our improved means for clamping and reinforcing the leg laminations and tying the top and bottom yoke sections together, we have illustrated the core with the windings removed. The location of the windings on the core legs is indicated at FIGS. 1 and 2 by phantom lines.

In order to preclude the imposition of tensile forces on the transformer legs when the transformer is lifted from the top, as by grasping the lugs 14, or when the core is subjected to spreading forces under short circuit conditions, it is necessary to tie together the top and bottom yokes 4 and 5 by some means other than the leg laminations themselves. It is also necessary to clamp the leg laminations together transversely and to reinforce and tie together the separate halves of each leg section when a transverse cooling slot such as the slot 8 is provided through the legs. All these functions are served by a pair of elongated reinforcing plates 15 on opposite sides of each leg, one of which is shown separately at FIG. 3. The reinforcing plates 15 lie in flat-wise engaging relation against each outer leg lamination and extend from the bottom yoke .5 to the upper yoke 4. At their ends the plates 15 are provided with outwardly extending pins 16 which engage cooperating holes in the yoke clamps 10, 10a and 11, 11a. If desired, of course, the reinforcing plates 15 may be provided with holes rather than the pins 16 to the yoke clamps by means of bolts or by pins on the clamps. In end-to-end tension, then, the plates 15 serve to connect the upper and lower yokes together independently of the leg laminations and to relieve the leg lamination-s of any tensile stress arising from lifting or short circuit forces.

Each reinforcing plate 15 is formed of a flat elongated bar slotted longitudinally along its center, as at 17, to form a pair of parallel longitudinal legs 18. The slot 17 extends for substantially the full length of the plate 15 (and the adjacent leg section) and terminates just sufficiently short of each end to form at each end an integral connecting lug 19. The slot 17 serves to break up most eddy current paths in the plate 15 for a purpose which is illustrated at FIG. 2. At FIG. 2 I have indicated by means of fragmentary flux lines 20' the direction of certain leakage flux which unavoidably escapes along the length of each core leg without entering the yokes 4 and 5. Such leakage flux escapes around the entire periphery of each core leg, but that portion of it in planes generally transverse to the planes of the laminations passes in part through the reinforcing plates 15. The radial component of such escaping leakage flux sets up eddy currents in the plates 15 which would heat a solid plate to an undesirably high temperature because of its relatively low resistance. By slotting the plate 15 longitudinally, the amount of flux linking any eddy current path per unit length of path is decreased, and as a result eddy current density and power loss in the plate is diminished. The loop consisting of the two legs 18 of the plate and the upper and lower lugs 19 does not link any leakage flux because the flux at top and bottom is oppositely directed and therefore cancels any inductive effect in the loop. While I have shown only a single wide central slot 17, it will of course be evident to those skilled in the art that, if desired, additional longitudinal slots may be provided in each of the leg portions 18 of the reinforcing plate in order to further reduce undesirable eddy currents.

In order more effectively to utilize the reinforcing plate 15 in clamping together the core leg laminations, each such plate includes a plurality of rigid cross arms 21 extending across the lengthwise legs 18 and the slot 17. Each cross arm is welded or otherwise fixedly and conductively connected to one of the legs 18 and separated from the other leg 18 by a strip of insulating material 22 (FIGS. 3, 4 and 5). Preferably one group of alternate cross arms 21 is welded to one of the legs 18 and the remaining group of alternate cross arms is welded to the other leg 18. To serve as clamps for the leg laminations the cross arms 21 are each provided at their center and within the slot 17 with a bolthole. Opposite pairs of cross arms 21 are drawn together in clamping relation by transverse clamping bolts 23- which pass through the leg slots 8.

The assembly of the reinforcing plates 15 to the trans.-v former legs is most clearly illustrated at FIGS. 1 and 2. From FIG. 2 it will be observed that the cross arms 21 are placed on the inner side of the lengthwise legs 18 and thus serve to space the legs 18 away from the adjacent core lamination. As shown in FIG. 5, the cross arms 21 lie flat against the outer leg laminations and are clamped together by the bolts 23. If desired, a thin sheet of insulating material (not shown) may be inserted between the cross arms 21 and the leg laminations. At the upper and lower ends of each core leg a flat insulating spacer 25 is inserted between the leg laminations and the plate 15 in order to wedge the leg laminations tightly together. In assembly the insulating strips 25 at the lower ends of the core legs are inserted as the lower yoke 4 and the legs 1, 2 and 3 are assembled while laying fiatwise on a horizontal surface. After the core is moved to an upright position and the coils assembled on the legs, the upper yoke laminations are assembled between the upper ends of the reinforcing plates 15. For this operation it is desirable to have available the slight additional space provided by the location of cross arms 21 on the inner side of the slotted plate 15. After the yoke laminations are assembled, similar insulating spacers 25 are inserted between the upper ends of the plates 15 and the upper yoke laminations in order to wedge these laminations tightly in place. The assembly is then completed by positioning the upper yoke clamps 10, 10a and bolting up the upper yoke between them.

At FIG. 6 we have shown in fragmentary cross sectional view an alternative assembly of our improved reinforcing plate. In this embodiment the entire plate 15 is insulated from the core laminations by an insulating sheet 30 and the cross arms 21 are on the outer side of the plate and insulated from both plate legs 18 by insulating strips 31. As shown in plan view at FIG. 7, the cross arms 21 may be shorter than the full width of the plate 15.

It will be understood of course that while the form of the invention herein shown and described by way of illustration constitutes a preferred embodiment of the invention, other embodiments will occur to those skilled in the art. It will be understood that the words used are words of description rather than limitation, and that various changes and modifications may be made without departing from the spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A transformer core comprising a plurality of leg sections formed of stacked laminations of magnetic material, a pair of yoke sections formed of stacked laminations of magnetic material and disposed in substantially parallel spaced relation across juxtaposed ends of said leg sections, a pair of elongate yoke clamps extending along opposite sides of each said yoke section in clamping relation with the laminations thereof, a pair of elongate reinforcing plates extending in clamping relation along opposite sides of at least two said leg sections and connected at opposite ends to said yoke clamps, each said reinforcing plate comprising a flat bar slotted longitudinally for substantially the full length of the adjacent leg section and a plurality of transverse cross arms fixed to said bar, and clamping bolts extending between aligned cross arms on opposite sides of the same leg section.

2. A transformer core according to claim 1 in which said cross arms are electrically insulated from said slotted bar at at least one side at the longitudinal slot.

3. A transformer core according to claim 1 in which said cross arms are disposed between the outer leg laminations and said slotted bar, and insulating spacers are interposed in Wedging relation between the yoke laminations and the ends of said slotted bar.

4. A transformer core according to claim 2 in which said leg sections are slotted longitudinally transversely of the laminations and said clamping bolts traverse such slots, whereby both transversely divided halves of each leg are clamped between opposite cross arms of a single pair of said reinforcing plates.

References Cited UNITED STATES PATENTS 7/1925 Burnham 336210 X 10/1959 Wilk et al 336-210 LARAMIE E. ASKIN, Primary Examiner. H. W. COLLINS, D. A. TONE, Assistant Examiner. 

1. A TRANSFORMER CORE COMPRISING A PLURALITY OF LEG SECTIONS FORMED OF STACKED LAMINATIONS OF MAGNETIC MATERIAL, A PAIR OF YOKE SECTIONS FORMED OF STACKED LAMINATIONS OF MAGNETIC MATERIAL AND DISPOSED IN SUBSTANTIALLY PARALLEL SPACED RELATION ACROSS JUXTAPOSED ENDS OF SAID LEG SECTIONS, A PAIR OF ELONGATE YOKE CLAMPS EXTENDING ALONG OPPOSITE SIDES OF EACH SAID YOKE SECTION IN CLAMPING RELATION WITH THE LAMINATIONS THEREOF, A PAIR OF ELONGATE REINFORCING PLATES EXTENDING IN CLAMPING RELATION ALONG OPPOSITE SIDES OF AT LEAST TWO SAID LEG SECTIONS AND CONNECTED AT OPPOSITE 